Various optical/flow systems employed for transporting a fluid within an analytical instrument to an imaging and optical analysis area exist in the art. A fluid sample is typically delivered into the bore of a flow chamber and the sample is interrogated in some way to generate analytical information concerning the nature, content or properties of the sample. For example, the system may automatically image the sample as it is flowing through a chamber containing the fluid under analysis.
Automatically imaging passing particles in fluid flow by illuminating the fluid and activating an image capturing device, such as a camera with a frame grabber, at regular intervals, can provide images which contain a lot of information about the particles. However, other measurement techniques can be more useful. One such technique of interest is light obscuration. The light obscuration technique, which is sometimes referred to as single particle optical sensing, involves the back illumination of a fluid, with a laser such that the loss of light due to the particle can be detected by a single photodiode. In some situations, detection of light loss is easier and more accurate than detection by light scatter. Therefore, there is a need in the art for an effective way to provide light obscuration measurements as well.